Conventionally, members made of aluminum or aluminum alloy, such as a variety of exterior parts and structural parts including pistons and cylinders of internal combustion engines and hydraulic and pneumatic pistons and cylinders, have been anodized to form an anodized film (anodic oxide coating) on the surfaces of the members for the purpose of improving the corrosion resistance and wear resistance or coloring.
For this anodizing treatment, for example, as disclosed in JP2002-47596A, electrolytic treatment is performed by applying DC voltage, AC voltage, AC and DC superimposing voltage, or pulse voltage to between a workpiece (anode) and a cathode in the state in which the workpiece is immersed in an electrolytic solution. The present inventors discovered a method for forming a high-quality anodized film at a high speed, without being affected by an alloy component, including a treatment of repeating the anodization by applying positive voltage for a very short period of time and removing charges from the film as disclosed in JP2006-83467A.
In the conventional anodization, it has been thought to be proper for treatment to be performed at a current up to about 3 A per 1 dm2 of the surface area of the workpiece to prevent burning. However, in the treatment method disclosed in JP2006-83467A, the temperature rise is restrained by the removal of film charges, and as a result, a current of 30 A or more per 1 dm2 of the surface area of a workpiece can be supplied in a positive voltage applying period, so that the treatment time can be shortened to a fourth to a fifth of the conventional treatment time.